A plasma reactor may be employed to perform various processes on a workpiece, such as a semiconductor wafer during the fabrication of microelectronic devices on the workpiece. The wafer is placed inside a vacuum chamber of the reactor and processing gases are introduced. The gases are irradiated with electromagnetic energy to ignite and maintain a plasma. Depending upon the composition of the gases from which the plasma is formed, the plasma may be employed to etch a particular material from the workpiece or may be employed to deposit a thin film layer of material onto the workpiece.
In the case where the plasma reactor is used for etching, examples of typical processing gases employed as etchants include Cl2, BCl3, CF4, SF6, NF3, HBr, and various CxHyFz gases, among others. These gases, however, are not chemically reactive enough in these forms to satisfactorily etch the materials desired to be removed from the workpiece undergoing processing in the reactor. This is where the plasma comes into play.
Process gases are irradiated with electromagnetic energy to ignite and maintain a plasma. Species of neutral and charged particles, as well as other particles and substances, are created in the plasma from the processing gases. For example, if Cl2 is used as the process gas, the following species of neutral and charged particles may be present in the chamber:
Cl2xe2x86x92Cl2+Cl+Cl++e+Clxe2x88x92+Cl2+
Neutrals formed from the etchant gases, such as Cl, F and Br, are extremely unstable and reactive, and can be effectively used to chemically react with materials on the workpiece to produce gaseous substances, thereby in effect removing or etching the material from the workpiece. For example, in the case where it is desired to etch silicon from the surface of a semiconductor wafer, Cl2 can be symmetrically dissociated to form Cl neutrals in the plasma. These Cl neutrals will react with the silicon of the wafer according to the following formula:
Si+XClxe2x86x92SiClx, X=2, 4 xe2x80x83xe2x80x83(1)
The product, SiClx is a gas that is eventually evacuated from the processing chamber.
The foregoing reaction, however, may not occur unless sufficient energy is added. This energy can be in the form of heat, but typically in a plasma assisted etch process, the majority of this energy comes from a physical bombardment of the surface of the wafer. The physical bombardment is the task performed by the charged particles also formed in the plasma from the processing gases. Typically, charged particles are drawn toward the wafer via a bias power applied to the wafer support to enhance the bombardment and create a desired directionality to itxe2x80x94usually normal to the upper surface of the wafer. These charged particles not only provide the energy that fuels the chemical etch process associated with the etchant gas neutrals, but also physically remove material from the surface of the wafer as a result of the particles impact with the wafer.
The charged particles need not be exclusively formed from etchant gases. Any charged particle can be made to bombard the wafer and create the desired effect regardless of whether it will chemically react with the material to be etched. For example, when more charged particles than can be obtained from the etchant gases alone are required for a particular etch process, a non-reactive gas such as argon may be introduced. The argon forms charged particles in the plasma. Although the introduced argon is not chemically reactive with the wafer materials, it provides the desired boost in the overall availability of charged particles used to bombard the wafer.
The concentration or density in the plasma of both the neutral particles formed from the etchant gases and the charged particles formed from all of the processing gases will play a significant role in the etching process and in determining the characteristics exhibited by the etched workpiece. For example, both act to etch material from the workpiece. Therefore, an increase in density of all will have the effect of increasing the overall etch ratexe2x80x94often a desirable effect.
It must be noted, however, that the physical bombardment of the workpiece by the charged particles will also etch materials that may not be intended to be removed. Thus, an increase in the charged particle density can result in damage to the devices being formed on the workpiece, even though the etch rate of the materials intended to be etched would increase. As a result, it can be more advantageous to increase the overall etch rate by increasing only the density of the etchant gas neutrals in the plasma.
The relative densities of the etchant gas neutral and the charged particle species formed in the plasma will also have profound effects, for example, on etch process characteristics such as etch selectivity, etch feature profile, and etch rate microloading.
The term etch selectivity refers to the ratio of etch rates of two different materials on a workpiece undergoing etching in the plasma reactor. To form features and patterns in the various layers of a workpiece, the etch process must be selective so that some materials are etched, while others are not. In one common scenario, it is desired that a silicon layer on a workpiece be etched much faster than photoresist or oxygen-containing layers of the workpiece so as to etch a pattern into the silicon. This is referred to as a high silicon-to-photoresist and silicon-to-oxide selectivity, respectively.
The following example of etching a hole through a silicon layer to an underlying gate oxide layer on a semiconductor wafer, illustrates one example of the importance of high selectivity. Prior to etching, a layer of photoresist material is formed over the surface of the silicon layer over those areas that are not to be etched. Accordingly, there is no photoresist formed in the area where the hole is to be etched. The desired result of the etching process is to quickly etch through the silicon layer where the hole is to be formed, but not to significantly etch the surrounding photoresist, or the underlying gate oxide layer. Thus, a high silicon-to-photoresist and silicon-to-oxide etch selectivity is desired. If an adequate level of selectivity is not maintained, a so-called xe2x80x9cpunch throughxe2x80x9d condition can result wherein the photoresist or oxide layer is etched through causing damage to the device being formed on the workpiece.
The densities of the plasma species have a significant impact on the selectivity exhibited during an etching process. For example, if the process chemistry is such that the etchant gas neutrals chemically react with the material to be etched (e.g. silicon) to a greater extent than other materials (e.g. photoresist and oxide), then having a high density of neutral species will help achieve the desired selectivity by causing an increased etch rate of the material being etched in comparison to the other materials. Conversely, since charged particles remove material from the workpiece surface through physical impact, they tend to etch all the various materials of the workpiece equally. Thus, a greater density of the charged particle species in the plasma can cause a greater etch rate of all the workpiece materials.
Accordingly, an increase in etchant gas neutral species under certain conditions can increase selectivity, while a decrease in the neutral species likewise can decrease selectivity. Whereas, an increase in the density of charged particle species under certain conditions can cause a decrease in selectivity, a decrease in charged particle species under certain conditions can cause an increase in selectivity. Therefore, one way of optimizing the desired selectivity of an etch process would be to increase the ratio of etchant gas neutral to the charged particle species in the plasma to the point where there are just enough of the charged particles to facilitate the reaction of the etchant gas neutrals with the material to be etched, but no more.
The etch feature profile exhibited by an etch workpiece will also depend heavily on the relative densities of etchant gas neutral and charged particle species in the plasma. The term etch feature profile refers the angle of a sidewall of a feature etched into a layer of material on the workpiece in relation to the surface plane of the workpiece. This angle can vary between a severe undercut, where the sidewall forms an acute angle with the surface plane of the workpiece, to a significant outward taper where the sidewall forms an obtuse angle with the surface plane. Typically, a straight profile is desired where the feature sidewall forms a 90 degree angle with the surface plane of the workpiece.
The undercut profile occurs when etchant gas neutrals, being chemically reactive with the material into which a feature is etched, cut into the material underneath the overlying photoresist layer down to an underlayer (such as the gate oxide layer in the previously-described silicon etch process). The greater the density of the etchant gas neutral species, the greater the undercut. Often the undercutting potential of the etchant gas neutrals is mitigated by the deposition of a passivation material onto the material being etch. Essentially, the passivation material, which is typically formed in the plasma, deposits on the surfaces of the workpiece and has the effect of resisting etching by the etchant gas neutrals. For example, in the silicon etch scenario described previously, oxygen or nitrogen is often introduced as part of the processing gases. The oxygen or nitrogen reacts with the silicon etched from the semiconductor wafer (or introduced into the plasma by other means) to form various silicon and oxygen or silicon and nitrogen containing materials, respectively. These materials deposit in varying degrees onto the surface of the wafer and resist the etching effects of the etchant gas neutrals. Due to the typical chemistries involved and the action of the bombarding charged particle species, the passivation materials tend to form more readily on the sidewalls of a feature formed in the material being etched, than on the bottom of the feature. This results in a lower etch rate of the feature sidewall, and so a decreased tendency to form an undercut profile. If the etchant neutral density is too high, however, this passivation process may be insufficient to prevent an undercut etch profile.
Charged particle species can have just the opposite effect on the etch feature profile. An increase in the density of charged particle species in the plasma can have create the aforementioned outwardly tapered profile. This can occur as the impact of the charged particles bombarding the bottom of a etch feature removes material that can subsequently redeposit on the sidewalls of the feature. In this way, the outwardly tapered profile can occur.
Thus, one way of creating the desired etch profile angle would be to balance the relative densities of the etchant gas neutral and charged particle species within in the plasma. In other words, the concentration of etchant gas neutral and charged particle species would be made such that the desired etch profile is produced given the opposing effects each has on the profile.
The relative densities of etchant gas neutral and charged particle species in the plasma also have an effect on the etch rate microloading exhibited by an etched workpiece. Etch rate microloading refers to the phenomenon wherein the etch rate tends to be different in areas of dense, closely-spaced etch features on the workpiece than in areas with more widely-spaced features. This can result in an undesirable non-uniformity in the etch depth of features formed in the layer of material being etched. Etch rate microloading is a complex phenomenon. It is known, however, that varying the ratio of etchant gas neutral to charged particle species in the plasma results in a change in the severity of the etch rate microloading.
Accordingly, the ability to vary the densities of the etchant gas neutral and the charged particle species within in the plasma could be advantageously used to ameliorate the etch rate microloading effect.
As can be surmised from the forgoing background, the ability to control the densities of the etchant gas neutral and charged particle species existent in the plasma is very desirable. Establishing a particular neutrals-to-charged particles ratio, for example, can optimize such process characteristics as etch rate, selectivity, etch feature profile, and etch rate microloading. Furthermore, specific charged particle species ratios and neutral species ratios may even further enhance the aforementioned, or other, process characteristics.
Until now, however, the relative densities of the particle species formed within a plasma have been inexorably tied together because, among other limitations, the dissociation rate (i.e. the rate at which neutrals are generated in the plasma) and the ionization rate (i.e. the rate at which charged particles are generated in the plasma) are both dependent upon the level of power coupled into the processing chamber. The density of the etchant gas neutrals increases with increasing power input into the reactor. Unfortunately, any increase in the power input also increases the density of charged particles in the plasma. Likewise, the densities of the different neutral particles, or of the different ionized particles are coupled. As discussed above, many advantageous etch characteristics are dependant upon creating a particular ratio between the densities of the etchant gas neutral and charged particle species, including the ratio between different neutral particle species, or between different charged particle species. This often may entail increasing the density of one species while decreasing the density of the anotherxe2x80x94something that cannot be accomplished with current plasma reactors and etch processes.
With current reactors, charged particle and neutral particle species densities may be controlled by adjusting reactor parameters, such as source power, chamber pressure, and temperature. Adjusting these parameters, however, affects plasma density and plasma ion energy. Therefore, plasma density and plasma ion energy are coupled to species densities. In reactors that employ only a capacitive power source to generate and sustain the plasma, the three characteristics-plasma density, ion energy, and species densities, are coupled together. For example, increasing source power to increase plasma density correspondingly increases ion energy and the charged-to-neutral particle density ratio. Therefore, adjusting source power or pressure to change one of the characteristics affects the other two.
Other reactors, such as an inductively coupled plasma reactor, which utilize two power sources to control plasma characteristics, provide greater control of plasma characteristics. Using a second source, such as an inductively coupled power source, along with a capacitive power source allows plasma density and plasma ion energy to be decouple. In other words, plasma density and ion energy may be separately controlled. In these reactors, however, species densities are still coupled to plasma density and ion energy.
Furthermore, neither type of reactor allows control of the density of the specific species of charged or neutral particles within the plasma, such as the ratio of Cl+to Cl2+. Control of the densities of specific species of charged or neutral particles would provide even greater control of etch or deposition results thereby improving workpiece processing.
Accordingly, there is a need for a plasma reactor design and etch process that decouples control of plasma species densities, thereby allowing the ratio of these densities to be manipulated so as to optimize process characteristics such as etch rate, selectivity, etch feature profile, etch rate microloading, and more.
The foregoing problems are solved in the apparatus described in U.S. application Ser. No. 09/119,417 referred to above in which control of neutral species density and control of ion density are decouple, permitting the process designer to decrease ion density while increasing neutral species density. The apparatus employs, in addition to the reactor chamber, a secondary or neutral source chamber in which a plasma is generated consisting of the desired species, and only the neutrals are extracted from this plasma and provided to the main reactor chamber. The neutral species density in the main reactor chamber is increased without increasing the ion density in the main chamber by increasing the source power in the secondary chamber.
The degree to which the main chamber neutral species density increases with secondary chamber source power is a measure of the degree to which control of the ion density has been decoupled from control of the neutral density in the main chamber.
The present invention concerns a method for operating the dual chamber apparatus described in the referenced application. It is a discovery of the invention that the decoupling of the neutral particle density control from the ion density control depends upon process conditions, certain process regimes optimizing this decoupling and others inhibiting it.
The invention is a method for controlling and maximizing the degree to which control of the main chamber neutral particle density is decoupled from the ion density. In the invention, it was found that this decoupling increases as the main chamber source power is decreased. Conversely, this decoupling decreases with increasing main chamber pressure, and in fact can disappear entirely above a certain main chamber pressure. Therefore, the method in one embodiment of the invention consists of determining the desired increase in main chamber neutral particle density to be contributed by the secondary chamber for a given main chamber ion density range, and then to maintain the main chamber source power below a level beneath which the secondary chamber is capable of supplying the desired main chamber neutral density contribution.
In a related embodiment of the invention, the method consists of determining the desired rate of increase in main chamber neutral particle density relative to secondary chamber source power, and then to maintain the main chamber source power below a level beneath which the secondary chamber is capable of meeting this rate of increase.
It is a further discovery of the invention that the neutral density contribution from the secondary chamber is scaled by the secondary chamber source power, the main chamber pressure and the main chamber gas inlet rate of an electron temperature-reducing molecular gas additive. Thus, in a further aspect of the invention, the main chamber neutral density contribution from the secondary chamber is augmented by increasing the secondary chamber source power, by decreasing the main chamber pressure and/or by increasing the gas inlet rate of the electron temperature-reducing molecular gas additive to the main chamber.